1. Field of the Invention
This invention relates to a disc motor circuit in a disc playback device for playing back discs such as an optical type video disc and a Compact Disc in the Compact Disc Digital Audio System.
2. Description of the Related Art
The control of rotation of a disc motor in a disc playback device is generally made by switching between an AFC (auto frequency control) and a PLL (phase locked loop).
The AFC is a rough control according to which rotation of a disc motor is controlled by comparing, in frequency, a pulse generated from a frequency generator, i.e., a device generating a pulse with a period corresponding to the rotation speed of the disc motor, with a reference clock. The AFC can be performed even when no synchronizing signal is obtained from a signal reproduced from a disc.
The PLL is a control of a relatively high precision according to which rotation of a disc motor is controlled by comparing, in phase, a synchronizing signal taken out of a signal reproduced from a disc (e.g., a horizontal synchronizing signal) with a reference clock.
When a synchronising signal cannot be taken out of a signal reproduced from a disc because the disc rotation speed is largely deviated from a specific speed, e.g., at the time of starting the disc motor, the AFC is carried out and, when the disc rotation speed has entered a range in which the synchronizing signal can be taken out of the reproduced signal from the disc, the control is switched to the PLL.
The PLL can be implemented within a certain range of the rotation speed above and below a target rotation speed and, accordingly, switching to the PLL does not necessarily bring about a stable rotation of the disc motor at the target speed immediately.
Besides, even after a stable rotation at the target speed has been attained by the PLL, the PLL sometimes is unlocked for some reason such as a shock applied to the disc from outside and a scratch on the disc and the disc rotation thereby becomes unstable. If, accordingly, controls which should be carried out in a stable rotation state of the disc such as a jitter removal control and a control for switching a window for detecting a horizontal synchronizing signal are carried out on the assumption that the stable rotation state has been brought about merely because the control mode has been switched to the PLL, accurate implementation of these controls cannot be expected.